nix-super/doc/manual/opt-verbose.xml

<varlistentry>
  <term><option>--verbose</option></term>
  <listitem>
    <para>
      Increases the level of verbosity of diagnostic messages printed
      on standard error.  For each Nix operation, the information
      printed on standard output is well-defined; any diagnostic
      information is printed on standard error, never on standard
      output.
    </para>

    <para>
      This option may be specified repeatedly.  Currently, the
      following verbosity levels exist:
    </para>

    <variablelist>
      <varlistentry>
        <term>0</term>
        <listitem>
          <para>
            <quote>Errors only</quote>: only print messages explaining
            why the Nix invocation failed.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>1</term>
        <listitem>
          <para>
            <quote>Informational</quote>: print
            <emphasis>useful</emphasis> messages about what Nix is
            doing.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>2</term>
        <listitem>
          <para>
            <quote>Talkative</quote>: print more informational messages.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>3</term>
        <listitem>
          <para>
            <quote>Chatty</quote>: print even more informational messages.
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>4</term>
        <listitem>
          <para>
            <quote>Debug</quote>: print debug information:
          </para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>5</term>
        <listitem>
          <para>
            <quote>Vomit</quote>: print vast amounts of debug
            information.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>

  </listitem>
</varlistentry>